U[sbnd]Pb, Pb[sbnd]Pb and Sm[sbnd]Nd dating of authigenic monazite: implications for the diagenetic evolution of the Welsh Basin

Abstract

Authigenic monazite nodules from Silurian turbiditic rocks in the central Welsh Basin are zoned with respect to Nd isotope composition. The cores are relatively radiogenic, and the rim-to-core trend projects toward the composition of associated, very early authigenic, apatite concretions. These authigenic phases therefore equilibrated with changing fluid composition as they formed. The apatites and the monazite nodule cores may have included significant Nd from organic matter or seawater, whereas the REEs in the outer parts of the monazite nodules were derived from clastic detritus. The apparent SmNd ‘age’ obtained from these phosphates is a mixing line, which gives an Ordovician age for these Silurian rocks. The UPb isotope system does not show similar trends, and shows promise as an alternative method for dating such phosphates: a date of 417 ± 11 Ma 2σ (MSWD = 107) agrees well with recent estimates of the time of the diagenetic, smectite-illite transition in the rocks of this study. A PbPb isochron gave an age of 422 ± 24 Ma 2σ (MSWD = 0.9) this shows that the Pb is in isotopic equilibrium and is undisturbed, and that the elevated MSWD of the UPb regression is caused by slight subsequent mobility in the UPb system. The event dated by the monazite is probably compactional dewatering during accelerated mid-Silurian sediment loading in the central Welsh Basin.